This study examines child psychologists' use of the DSM-IV when making diagnostic decisions. We will test whether clinicians follow DSM-IV rules or utilize internally-held theories by asking participants to create individual theory maps representing each of four specified DSM-IV disorders and then to assess clinical vignettes portraying children with these same disorders. We will further gauge the impact of internalizing and externalizing symptoms on clinicians' application of rules and theory during diagnosis. Finally, we will investigate how discrepant data from multiple sources influence clinician's decision-making processes. This study is unique in (1) application of an experimental technique not utilized, to date, with a population of child psychologists, (2) synthesis of the respective contributions of the literature on clinical decision-making and categorization to further the understanding of the diagnostic behavior of child clinicians. This knowledge can ultimately be utilized in systematic efforts to improve diagnostic reliability and consequently advance the dissemination of empirically validated treatments to appropriately identified patients in outpatient clinical settings.